From the technology of the GSM (Global System for Mobile telecommunications) there is known the concept of a slow associated control channel or SACCH. The SACCH is a logically separate channel that physically shares a capacity allocation with a dedicated traffic channel. In the following we describe briefly some known features of the SACCH and its relation to the corresponding traffic channel.
In GSM, capacity at the radio interface is allocated in burst periods or BPs, which are sometimes also referred to as slots. Eight consecutive burst periods constitute a frame, the length of which in time is 60/13 ms or approximately 4.615 ms. A full-rate traffic channel corresponds to a capacity allocation where exactly one burst period is allocated from each frame. The ordinal number of the allocated burst period stays the same from frame to frame and is even the same in the uplink and downlink frames that constitute the parts of a bidirectional full-rate traffic channel. A fixed shift of three burst periods exists between the uplink and downlink frames so that the uplink burst period allocated to a certain bidirectional full-rate traffic channel always occurs three burst periods after the correspondingly numbered downlink burst period.
The full rate should not be confused with the maximum achievable data rate, because in the later development work concerning GSM also so-called multislot channels have been specified where more than one burst period per frame are allocated to a single communication connection. Instead, a traffic channel with one slot per frame allocated thereto could be regarded to have a “nominal” full rate.
A period of exactly 120 ms comprises 26 frames and is denoted as the multiframe. Of the 26 allocated burst periods in a multiframe, 24 are used for the traffic channel, one is used to transmit an SACCH burst and one is a so-called idle burst period where nothing is transmitted.
In order to understand the relation between an SACCH burst and certain SACCH bits to br transmitted one must be familiar with some channel encoding. A sequence of 184 uncoded SACCH bits is known as an SACCH block. In the channel encoding process it is first subjected to encoding by a Fire code, which adds 40 bits. The result is convolutionally encoded with a rate ½ convolutional encoder which gives at its output an encoded SACCH block consisting of 456 bits. These are distributed into four chunks of 114 bits each, so that eventually four complete SACCH bursts are needed in order to get the information contents of a single SACCH block transmitted. A simple multiplication shows that since one SACCH burst is transmitted within each multiframe of 120 ms, the time required to get the information contents of a single SACCH block transmitted is four multiframes or 480 ms.
Already the original form of the GSM specifications defined also other types of traffic channels than the full-rate one referred to above. More specifically half-rate and eighth-rate traffic channels have been defined. The burst period allocations for these follow a simple alternation scheme where e.g. two half-rate traffic channels take the place of a single full-rate one. Of the 24 traffic burst periods within a multiframe each half-rate channel gets 12. One SACCH burst period is still left for each half-rate channel so that during the SACCH burst period of the first half-rate channel the second is idle and vice versa.
The advent of EDGE (Enhanced Data Rates for GSM Evolution) has promoted the specification of a quarter-rate traffic channel which may be denoted as a TCH/4 for short. FIG. 1 illustrates a proposed allocation of traffic (T) and SACCH (S) burst periods for the four TCH/4s which in the allocation scheme take the place of a single full-rate traffic channel. The graphical representation selected for FIG. 1 is the known one where time advances “helically”, i.e. the stream of consecutive burst periods is wound into a helix where one round has the length of one frame (8 burst periods). This representation has the advantage of showing the cyclically repeated burst periods allocatable to one full-rate traffic channel in a straight row. We designate the four TCH/4s as the subchannels 0, 1, 2 and 3. In order to have exactly one fourth of the traffic capacity of a full-rate traffic channel left for each subchannel we must assume that of the 26 allocated burst periods of a multiframe, six must be allocated for each subchannel. However, this assumption leaves only two burst periods per superframe free for SACCH bursts. In order to accommodate one SACCH burst per subchannel we must now consider the combined length of two multiframes, which is 240 ms in time and consists of 52 consecutive frames. FIG. 1 shows a proposed allocation scheme for the burst periods where each subchannel gets approximately every fourth allocatable burst period (“approximately”, because at frames 12–13 and 24–25 there is a shift of one frame to accommodate the SACCH bursts) and the SACCH bursts of subchannels 0, 1, 2 and 3 go into the 12th, 25th, 38th and 51st allocatable burst period of the “double multiframe” respectively.
The proposed allocation scheme of FIG. 1 has the drawback of doubling the transmission delay of an SACCH compared to that available for the SACCHs associated with full- and half-rate traffic channels. Previously we noted that the SACCHs associated with full- and half-rate traffic channels get one burst period per a multiframe of 120 ms; since four bursts are required to transmit the information contents of a single SACCH block, the total transmission delay for an SACCH block is four multiframes or 480 ms. In the scheme of FIG. 1 every SACCH gets one burst period per a double multiframe of 240 ms; since four SACCH bursts are still required to transmit the information contents of a single SACCH block, the total transmission delay is four double multiframes or 960 ms.
One might consider rearranging the SACCH allocations so that of the 16 SACCH burst periods within a sequence of eight consecutive multiframes, one subchannel would get e.g. the 1st, 3rd, 5th and 7th burst period, another subchannel would get the 2nd, 4th, 6th and 8th burst period, still another subchannel would get the 9th, 11th, 13th and 15th burst period and the remaining subchannel would get the 10th, 12th, 14th and 16th burst period. This way the group of four SACCH bursts required to transmit a single SACCH block would in every subchannel be transmitted within the period of four consecutive multiframes. However, after such a transmission there would be complete SACCH silence on that subchannel for another four consecutive multiframes, which is not advantageous. Another straightforward solution would be to allocate more than two burst periods per multiframe for SACCH, but this approach has the serious drawback of reducing the number of burst periods available for actual traffic and requiring fundamental changes to overall allocation schemes.